The above-mentioned treatment can advantageously be applied locally, i.e., in order to partially or completely dissolve the oxide layer in specific regions of the SeOI structure, which correspond to a desired pattern, while at the same time preserving the oxide layer in the other regions of the initial oxide layer.
Reference is then made to a “local dissolution” of the oxide layer.
An SeOI structure can thus be obtained having an oxide layer with varying thicknesses (in the case of partial dissolution), or else a hybrid structure, i.e., comprising both “SeOI” areas, in which the oxide layer has been preserved, and “bulk” layers, in which the oxide layer has been completely dissolved.
Such a structure can be used for the manufacture of different types of electronic components (e.g., “memory” components and logic components), which are normally manufactured on different supports.
As a matter of fact, the manufacturers of microprocessors have each developed technologies for manufacturing logic and memory components, however these two types of components are generally manufactured on different respective supports (i.e., a bulk or else an SeOI substrate).
In addition, shifting from one type of support to the other involves significant changes in manufacturing technology.
Therefore, the desirability of local dissolution lies in providing a microprocessor manufacturer with a plate comprising “bulk” and “SeOI” areas on which they will be able to manufacture both “logic” and “memory” components, while at the same maintaining the technologies in which they are skilled.
The precision of the local dissolution technique does indeed enable the “bulk” and “SeOI” areas of the components to be controlled.
Local dissolution can be implemented by forming a mask at the surface of the thin semiconductor layer, and by applying the heat treatment promoting oxygen diffusion.
Since the mask is made in a material forming an oxygen diffusion barrier, the oxygen can only diffuse through the exposed areas of the thin semiconductor layer, i.e., the areas that are not covered by the mask.
However, the disappearance of the oxygen atoms beneath the semiconductor layer results in a subsiding of the surface of the semiconductor layer.
Thus, in the case where the thin semiconductor layer is made of silicon, the following two phenomena are observed during the dissolution treatment:                on the one hand, the disappearance of the oxygen from the oxide layer, which is due to the diffusion of the atoms through the thin semiconductor layer; this phenomenon contributes to a subsidence of approximately half the thickness of the oxide layer. This value is associated with the existing ratio of 0.46 between the volume of the Si and the volume of the SiO2;        on the other hand, the disappearance of silicon from the surface of the thin semiconductor layer, which is due to the incorporation of highly volatile SiO complexes into the dissolution treatment atmosphere. This phenomenon contributes to a subsidence of the thickness of the oxide layer. As a matter of fact, a pair of O2 atoms causes the disappearance of two Si atoms.        
In all, the combination of these two phenomena results in a subsidence of the order of 1.5 times the thickness of the oxide layer.
The non-planarity of the surface of the semiconductor layer is detrimental to the subsequent formation of components.
This topography can be observed in FIG. 1, which shows a structure resulting from a local dissolution treatment.
This structure includes a substrate 1, an oxide layer 2, which has been dissolved in one region 2a, and a thin semiconductor layer 3, which is covered in places by a mask 4.
In the exposed area 3a of the semiconductor layer, the free surface has a difference in level in relation to the upper surface of the areas covered by the mask 4.
These topographical defects are detrimental to the manufacture of components on the thin semiconductor layer 3.
To that end, after deposition of the mask, document JP 2006-49725 anticipates carrying out a step for silicon epitaxy on the silicon surface not covered by the mask.
This additional thickness of silicon in the exposed areas compensates for the subsidence due to the dissolution of the oxide.
However, this step proves to be costly and has a negative impact on the treatment process.
In addition, it is difficult to anticipate mechanical-chemical type polishing aiming to planarize the surface in order to prevent the differences in level associated with the subsidence of the semiconductor layer, because this would remove a considerable degree of thickness of the semiconductor layer, the initial thickness of this layer being chosen to be thin in order to facilitate diffusion of the oxygen.
Furthermore, polishing tends to damage the uniformity of thickness of the semiconductor layer.
In addition, such polishing would risk polluting the surface of the thin layer, which is sought to be avoided when manufacturing electronic components.
Therefore, one of the objectives of the invention is to define a more economical and easy to implement process, thereby making it possible to minimize the subsidence of the surface of the semiconductor layer, so that, upon conclusion of the treatment, the smoothest possible surface is obtained.